I. Field of Invention
This invention relates generally to airfoils having adjustable aerodynamic shapes and, particularly, to an airfoil having an expandable surface which, in conjunction with the lowering of leading and trailing edge flaps, is effective to lower the landing speed of high performance craft.
II. Description of a Prior Art
It is well known that the maximum lift coefficient of a highly cambered airfoil is greater than that of a flat, thin one. Safety in landing an aircraft requires a low landing speed which is obtainable by using wings with high maximum lift coefficients. At the same time, high maximum speeds can only be obtained by using a thin, low-drag type of airfoil. This situation is aggravated when the aircraft being considered is a high performance aircraft such that low speed operation accounts for a very small, albeit extremely important, time-wise, portion of the operation of the aircraft.
A plethora of devices have been devised over the years for lowering the landing speed of high performance aircraft and thereby increasing their low speed maneuverability and safety of operation. In some instances, such attempts have been directed toward boundary layer control; that is, control of the layers of air which are intimate or in immediate contact with the outer surfaces of the airfoil. Thus, airfoils have been modified to incorporate slots or guides for directing airflow in a particular manner. Also, devices for blowing air over the surfaces of the airfoil or for drawing air away from the surfaces of the airfoil have been utilized. Numerous other devices have been devised or attempted, many of which have been complex or inefficient in their intended function or utilization of energy.
Expedients for lowering landing speeds which have been used successfully and, indeed, are standard on substantially all high performance aircraft include leading and trailing edge flaps. When raised, the flaps customarily blend with the contour of the remainder of the airfoil resulting in low camber and minimum drag for high speed flight. When lowered, the flaps increase the camber of the airfoil and provide the airfoil with maximum lift.
Unfortunately, one draw-back of such flaps resides in the transition angle which occurs at the interface between a flap and the main supporting section of the airfoil. Specifically, the planes of the upper surfaces of the flaps, when lowered, are angularly disposed relative to a plane of the upper surface of the airfoil's main supporting section. This situation causes undesirable turbulence across the upper surface of the airfoil which significantly detracts from its lifting capabilities.